It is known in the art that powder coating compositions are useful in painting metallic substrates. Such coating compositions are extremely desirable since their use greatly reduces and can even eliminate organic solvents in liquid paints. When powder coating compositions are cured by heating, little, if any, volatile material is given off to the surrounding environment. This is a significant advantage over liquid paints in which organic solvent is volatilized into the atmosphere when the paint is cured by heating.
For exterior durability, the powder coating composition of the prior art were formulated with a carboxylic acid group-containing polyester or acrylic polymer, a dicarboxylic acid group-containing material and beta-hydroxyalkylamide curing agent. These powder coating compositions and curing systems were said to be stable and possessed good physical properties. It is also equally well known in the art that the curing mechanism of the powder coating in an important aspect of powder coatings compositions. The curing system of this invention offers improved optical properties over prior art systems, while maintaining other desirable physical properties.
The curing mechanism of this invention is based on an acid group-containing polyester polymer, a beta-hydroxyalkylamide curing agent and a monocarboxylic acid group-containing material, which improves optical properties without adversely effecting other physical properties such as flexibility, durability, solvent and corrosive resistance.